The present invention relates to a two stage laser system in which two or more lasers including oscillator laser and amplifier laser are used, and more specifically, to a two stage laser system such as an injection locking laser system or an MOPA system, wherein a desired spectral line width or a spectral purity can be obtained at high output even when the integrated spectral characteristic of oscillator laser does not have a desired spectral line width or a spectral purity.
As the integrated circuits become finer and higher integrated, improvement of resolution has been demanded in exposure devices for use in fabrication of the integrated circuits. Consequently, the wavelength of exposure light emitted from the exposure light source has been driven to shorter wavelength, and exposure light sources such as KrF laser device with a wavelength of 248 nm or ArF laser device with a shorter wavelength of 193 nm have begun to be utilized as exposure light source for semiconductor, in place of conventional Hg lamp.
Further, in the exposure technique to realize semiconductor integrated circuit with a line width of 70 nm or narrower on semiconductor base, an exposure light source having a wavelength of 160 nm or shorter is demanded. At present, an F2 (fluorine molecule) laser device emitting ultraviolet light with a wavelength around 157 nm is regarded as promising light source for this purpose.
The F2 laser device (λ1=157.6299 nm, λ2=157.5233 nm: Sov. J. Quantum Electron. 16 (5), May 1986) has a spectral FWHM (Full Width at Half Maximum) of about 1 pm. The intensity ratio of the above two oscillation lines I(λ1)/I(λ2) is about 7. Typically, the oscillation line with the wavelength λ1(=157.6299 nm) having larger intensity is used in the exposure.
Exposure technique is roughly divided into two types as below.
1) Dioptric System
2) Catadioptric System
As to the catadioptric system, occurrence of the chromatic aberration is suppressed when this type of optical system is used in the exposure technique. Therefore, exposure device based on the catadioptric system is regarded as promising at present in the wavelength region around 157 nm. However, the catadioptric system has more difficulty in adjustment of the optical axis of exposure device in comparison with conventional dioptric systems.
On the other hand, the dioptric system is a projection optical system generally used in conventional exposure devices. In the exposure technique, the method to correct the chromatic aberration in the optical system is an important problem. In the dioptric system, the chromatic aberration correction has been achieved by combining a plurality of optical elements such as lenses with different refractive indexes. There is however limitation in available kind of optical material having transparency in the wavelength region around 157 nm, and the only available material is CaF2 (fluorite) at present.
Accordingly, a band narrowing is required to an F2 laser device which is used as light source for the dioptric exposure device. Specifically, it is required to band-narrow the spectral FWHM of the laser beam to a line width of 0.2 pm or narrower. In KrF laser device and ArF laser device, band narrowing is also required because laser beam of these devices has a spectral FWHM as broad as several hundred nm in free running operation without band narrowing.
Meanwhile, the average output required to an F2 laser device which is used as exposure light source is, for example, 20 W. Thus, when the repetition frequency of a laser device is 2 kHz, the pulse energy per pulse is 10 mJ, and when the repetition frequency of a laser device is 4 kHz, the pulse energy per pulse is 5 mJ.
However, when for instance an etalon is arranged within laser resonator as band narrowing means, it was difficult to have laser output of 5-10 mJ because of damage to coating on etalon, requiring the use of etalon without coating. This hindered the narrowing of the spectral line width. Further, abundant ASE (Amplified Spontaneous Emission) component limited the line width to over 0.4 to 0.6 pm. Consequently, it was difficult to achieve the band narrowing at a pulse energy level of 5 to 10 mJ.
Problems similar to the above also arise in KrF laser device and ArF laser device, when a high power is required.
In the abovementioned situation, in order to obtain a laser beam with a spectral line width of 0.2 pm or narrower and with a pulse energy of 5 mJ or higher, a two stage laser system comprising an oscillator laser and an amplifier laser may be employed.
Thus, an oscillator laser generates a laser beam having a spectral line width of 0.2 pm or narrower at a low output level, which is amplified in an amplifier laser to provide a laser beam having a spectral line width of 0.2 pm or narrower and a pulse energy of 5 mJ or higher. A similar construction may be employed also in KrF laser device and ArF laser device which are designed to have high power and narrow band.
Construction examples of two stage laser system include injection locking system and MOPA (Master Oscillator Power Amplifier) system. In construction of the injection locking system, a laser resonator is provided in the oscillator laser, and an unstable resonator is used in the amplifier laser. The construction of MOPA system does not have laser resonator in the amplifier laser. In the MOPA system having no laser resonator in the amplifier laser, the amplifier laser functions as a one pass amplifier of laser beam from the oscillator laser.
As band-narrowing element for the oscillator stage, a combination of prism group comprising one or more enlargement/dispersion prisms and grating, or a combination of etalon and total reflection mirror is used.
In FIG. 20 and FIG. 21, constructions of two stage laser systems according to conventional technique are shown. FIG. 20 shows a construction example of a MOPA system, and FIG. 21 shows a construction example of an amplifier laser in an injection locking system. For the oscillator laser in the construction of FIG. 21, a similar one as in FIG. 20 is used.
FIG. 20 schematically shows the device in top view. A laser beam emitted from an oscillator laser 10 functions as a seed laser beam in the laser system. An amplifier laser 20, 20′ has a function to amplify the seed laser light. Thus, the spectral characteristic of the entire laser system is determined by the spectral characteristic of the oscillator laser 10. The laser output (energy or power) of the laser system is determined by the output of the amplifier laser 20, 20′.
When a two stage laser system comprises an F2 laser device, chambers 1 of both oscillator laser 10 and amplifier laser 20, 20′ are filled with laser gas consisting of fluorine (F2) gas and buffer gas including helium (He) gas and neon (Ne) gas. When a two stage laser system comprises a KrF laser device, chambers 1 of both oscillator laser 10 and amplifier laser 20, 20′ are filled with a laser gas consisting of krypton (Kr) gas, fluorine (F2) gas and buffer gas including helium (He) gas and neon (Ne) gas. When a two stage laser system comprises an ArF laser device, chambers 1 of both oscillator laser 10 and amplifier laser 20, 20′ are filled with a laser gas consisting of argon (Ar) gas, fluorine (F2) gas and buffer gas including helium (He) gas and neon (Ne) gas.
The laser chamber 1 has a discharging part therein. The discharging part comprises a pair of electrodes 2, cathode and anode, arranged in line along the direction perpendicular to the paper surface. By applying a high voltage pulse to the pair of electrodes 2 from power supplies 7, 7′, a discharge occurs between the electrodes 2. In FIG. 20 and FIG. 21, only an upper electrode 2 is shown.
At both ends of the extension of light axis extending between the electrodes 2 provided in the chamber 1, window members 11 are provided respectively which are made of material transparent to oscillated laser light such as CaF2. Here, each window member 11 has a backside surface relative to the chamber 1 (outside surface), the two surfaces being arranged in parallel to each other and at a Brewstar angle in order to decrease a reflection loss.
In addition, a cylindrical fan (cross-flow fan) which is not shown in FIG. 20 and FIG. 21 is provided in the chamber 1 to circulate laser gas within the chamber 1 and to force laser gas through discharging part. Further, in the device of FIG. 20 and FIG. 21, both oscillator laser 10 and amplifier laser 20, 20′ have an F2 gas supply system and a buffer gas supply system to supply F2 gas and buffer gas to the chamber 1, and also have an exhaust system to exhaust laser gas from the chamber, but these are also not shown. Likewise, in case of a KrF laser device or an ArF laser device, Kr gas supply systems and Ar gas supply systems are provided, respectively.
The oscillator laser 10 comprises a band narrowing module 3 consisting of enlargement prism 4 and a grating (diffraction grating) 5. These optical elements in the band narrowing module 3 cooperate with a front mirror 6 to compose a laser resonator. Alternatively, a band narrowing module employing an etalon and a total reflection mirror in place of the enlargement prism and the grating may be used, but not shown. Further, as shown in FIG. 22, a band narrowing module 3 of a combined construction using a wave length selection module comprising an etalon 12 and a grating 5 may also be used.
The laser beam from the oscillator laser 10 (the seed laser beam) is guided and injected to the amplifier laser 20, 20′ by means of a beam transmission system including a reflection mirror which is not shown. In an injection locking system shown in FIG. 21, an unstable type resonator having, for instance, a 3× (three-power) magnification or larger is employed in the amplifier laser 20′ in order to allow even a low level input to be amplified.
A hole 13 is formed in a rear mirror 8 of the unstable resonator of the amplifier laser 20′ of the injection locking system. The seed laser beam passing this hole 13 is reflected as shown in FIG. 21. The injected laser beam is enlarged and effectively passes through the discharging part to have an increased laser beam power. The laser beam is finally emitted through a front mirror 9. An HR (High Reflection) coat is applied on the periphery of the rear mirror 8 which is provided at the center part with a spatial hole 13 as described above. The front mirror 9 has a HR coat applied on the center part of the convex mirror, and an AR (Anti Reflection) coat applied on a laser emitting part in the periphery.
A mirror base having an AR coat applied only on the hole part of a concave mirror may be used in place of the concave mirror 8 having the spatial hole 13. Further, an unstable resonator having no transparent part in the mirror may also be used.
In case of the F2 laser device, He gas, Ne gas, or a mixture of them is used as buffer gas in both the oscillator laser 10 and the amplifier laser 20, 20′. In addition, Xe gas may be added, if necessary.
A synchronous controller 21 controls the discharge timing of the oscillator laser 10 and the amplifier laser 20, 20′. First, it sends a trigger signal to the power supply 7 of the oscillator laser 10 as an ON command by which a high voltage pulse is applied to a pair of electrodes 2 of the oscillator laser 10 from the power supply 7. After a predetermined time interval, it sends a trigger signal as an ON command to the power supply 7′ of the amplifier laser 20, 20′.
The above described predetermined time interval means a time interval for synchronizing a timing at which the seed laser light is injected from the oscillator laser 10 to the amplifier laser 20, 20′ with a timing at which the amplifier laser 20, 20′ discharges.
As previously described, in the aforementioned two stage laser system, the spectrum characteristic of the entire laser system is determined by the spectrum characteristic of the oscillator laser. In an ultra narrow band oscillator laser having a line width of, for instance, 0.2 pm, the device has been conventionally constructed in such a manner that an integrated spectrum characteristic gives a desired characteristic (such as line width and purity).
In the following, the integrated spectrum characteristic will be described in outline. A laser beam emitted from the oscillator laser reciprocates within the laser resonator including the front mirror and the band narrowing module, until a part of it is extracted through the front mirror. Accordingly, the trailing half of the laser pulse has a narrower band than the leading half, since the trailing half reciprocates in the laser resonator more number of times, passing through the band narrowing module more number of times than the leading half. That is, the spectral waveform varies depending on time point within a laser pulse. Therefore, the spectrum characteristic has been evaluated by an integrated spectrum characteristic derived from a time-integration of each spectral waveform at each time point in a laser pulse.
In the situation as described above, in case of the F2 laser device, Ne gas or Ne rich mixture gas of Ne gas and He gas has been utilized as buffer gas contained in laser gas for the oscillator laser. This is due to the following reasons.
When He gas is used as buffer gas contained in laser gas for the oscillator laser, laser gain is larger than that in case of Ne gas, which leads to the generation of ASE. As the result, the ASE component is contained in the integrated spectral waveform of laser pulse emitted from the oscillator laser. Spectral line width (FWHM) of the ASE component depends on the laser gas total pressure and is 0.8 pm or larger. Since the ASE is a light emitted in one pass without reciprocating in the laser resonator, the ASE does not pass through the band narrowing module. Consequently, even if the band-narrowing of the laser beam emitted from the oscillator laser was carried out by means of the band narrowing module in the laser resonator, it was impossible to have a spectral line width (FWHM) of 0.8 pm or narrower in the integrated spectral waveform of a laser pulse because of the ASE component.
For this reason, mainly Ne gas has been used as buffer gas contained in laser gas for the oscillator laser, as it gives less laser gain and is less subject to ASE generation in comparison with He gas.
However, when Ne buffer gas is used, there arises a problem that high repetition rate is difficult. Due to a difference in gas characteristics such as discharge resistance, Ne gas is more subject to field concentration during discharge in comparison with He gas, which leads to an unstable discharge especially in high repetition rate. Further, since the Ne gas has heavier mass than that of He gas, it is difficult to allow high rate circulation of gas in the laser chamber by cross flow fan. Consequently, the gas replacement of the discharge region which is effected by gas circulation between discharges becomes insufficient in high repetition rate, which also leads to an unstable discharge.
Especially, a high repetition rate of 4 kHz or higher is difficult to be achieved with the present discharge technique in gas circulation system, discharge circuit, power supply, and the like.
Now, the inventors of the present application have found a relationship as described in the following between the light generated by laser gas excited by discharge between a pair of electrodes and the laser pulse rise occurring afterward (Japanese Patent Application No. 2002-46328).
FIGS. 23(a) and 23(b) are graphs each showing a laser pulse waveform, a sidelight waveform, and a spectral line width in a laser pulse in time sequence. FIG. 23(a) and FIG. 23(b) show different sidelight waveforms. The constructions of band narrowing module and laser resonator in both FIG. 23(a) and FIG. 23(b) are the same. Here, “sidelight” refers to a light generated by laser gas excited by discharge between a pair of electrodes. The observation of sidelight is performed from a position offset from the laser resonator (for instance, from an observation window arranged beside an electrode in a direction nearly perpendicular to the longitudinal direction of the electrode).
From the comparison between FIG. 23(a) and FIG. 23(b), it is obvious that the ASE component does not exist in the laser waveform when the sidelight rise (that is, the laser gain rise) is slow.
The reason why the ASE component is suppressed when the sidelight rise (the laser gain rise) is slow is thought to be as follows. In the case of FIG. 23(b), the sidelight rise (the laser gain rise) is so quick that the light generated by discharge is quickly amplified and is taken out of the laser resonator as ASE without reciprocation in the laser resonator, before it surpasses a predetermined threshold value to be taken out of the laser resonator as a laser beam.
On the other hand, in the case of FIG. 23(a), the sidelight rise (the laser gain rise) is delayed and the light generated by discharge is slowly amplified. The light reciprocates in the laser resonator without being amplified quickly and taken out of the resonator as ASE, and after it has grown up to surpass the threshold value, it is taken out of the resonator as laser beam. Thus, ASE component does not exist in the light emitted from the laser device.
It has become obvious from the comparison between FIG. 23(a) and FIG. 23(b), that ASE component does not exist when the sidelight rise (that is, the laser gain rise) is slow.
Here, by combining the time sequences of the spectral line width and the laser pulse waveform in FIG. 23(a) and FIG. 23(b), FIG. 24 is obtained.
FIG. 24 shows that the time sequences of the spectral line width of FIG. 23(a) and FIG. 23(b) are approximately on a same curve. The spectral line width of FIG. 23(a) at the time point where the laser pulse waveform of FIG. 23(a) rises up is substantially the same with the spectral line width of FIG. 23(b) at the same time point.
It is thought that this is because, until the light reciprocating in the laser resonator before the light generated after the start of discharge is taken out of the laser resonator (before the laser pulse rise), the light passes the band narrowing module several times and therefore laser beam is already band-narrowed by a certain degree.
Specifically, it is thought that the light generated at the start of discharge receives a band-narrowing and gradually decreases its spectral line width already before the laser pulse rise, and it continues to decrease gradually its spectral line width after the laser pulse rise until the laser pulse disappears. This phenomenon will be referred to hereinafter as line width characteristic. In addition, a time sequence of the spectral purity (purity characteristic) is also thought to have similar characteristic as line width characteristic, decreasing its width with the lapse of time.
It has been found from the experiments of the inventors of the present application that the line width characteristic and the purity characteristic mainly depend on the length of the laser resonator and band narrowing performance of the band narrowing module. Accordingly, it has been made clear that the spectral line width (integrated value) can be narrowed to a predetermined width and the spectral purity can be improved to a predetermined level, by controlling the sidelight rise (the laser gain rise) in such a manner that the laser pulse rise occurs after the light within the laser resonator has been band-narrowed to a predetermined spectral line width.
In addition, it has been made clear as is shown in FIG. 23(a) and FIG. 23(b), that the ASE generation can be suppressed by controlling the sidelight rise.
Based on the novel knowledge as described above, a conventional example will be discussed.
FIG. 25 is an illustration showing a relationship between a waveform of a band-narrowed laser pulse and a spectral line width variation (line width characteristic) of a light generated after the start of discharge (after the start of a sidelight emission), when the sidelight rise is regarded as the starting point (time origin). As described above, the decrease of the spectral line width with the lapse of time from the starting point of the sidelight is explained by the fact that the band narrowing proceeds even before the laser pulse rise because of frequent reflection of under-threshold laser gain in the band narrowing module.
For convenience of explanation, a time point after generation of the sidelight, at which point the line width falls below a desired width A is assumed as time point T1. In conventional technique, the spectral waveform was evaluated by the integrated spectral waveform. Accordingly, a laser pulse (generated after the time point T1) having a spectral line width narrower than A throughout the laser pulse was employed as seed laser beam in a two stage laser system. In this manner, a desired spectral characteristic (line width or purity) has been achieved.
As the time point T1 is determined depending on the line width characteristic regarding the starting point of the sidelight as time origin, T1 is determined by the discharge timing (starting point of the sidelight) of the oscillator laser. In conventional technique in which integrated spectral waveform was used, the laser gas condition, discharge circuit, or other conditions were adjusted so that the laser pulse always occurs after T1. Further, as described above, Ne gas or Ne rich mixture of rare gas was conventionally used as buffer gas because such knowledge as suppressing of ASE generation by delaying the sidelight rise was not available.
The synchronous controller stored data of a time interval between sending of trigger signal to the power supply of the oscillator laser and the laser pulse rise, and of a laser pulse width, and it controlled the discharge timing of the amplifier laser by sending trigger signal to the power supply of the amplifier laser in such a manner that the sidelight rise of the amplifier laser occurs (that is, discharge occurs) in a time interval during the oscillator laser pulse lasts (time interval TSYC).
Here, the time interval between sending of trigger signal to the power supply of the oscillator laser and the laser pulse rise, and the value of laser pulse width are obtained beforehand from such factors as state of laser gas and conditions of band narrowing module and discharge circuit.
In addition, it is supposed in the above description that laser pulse width and TSYC are substantially the same, but in practice the momentary laser power in the oscillator laser pulse is required to have such an intensity that a sufficient laser amplification can be performed in a two stage laser system. Accordingly, in the example of FIG. 25, the starting point of TSYC is later than the time point of laser pulse rise, and the end point of TSYC is earlier than the end point of the laser pulse.
As described above, in the past, the knowledge of delaying ASE generation by delaying the sidelight rise was not known, and the spectral waveform was evaluated by the integrated spectral waveform, which required the use of Ne gas or Ne rich rare gas as buffer gas. Therefore, it was difficult to achieve a high repetition rate pulse oscillation as discussed in the above.